The present invention relates to ways and means for building buildings predominately out of straw, and in particular to such buildings in which panels of compressed straw are united to serve not only a space-filling function, but also a load bearing function, so that the need for a building framework or skeleton whether internal to or external to the panelling is largely eliminated.
Building shelters, habitations and storage structures largely or partly of straw is a concept with an origin that predates recorded history. Straw/mud mixtures were used in the manufacture of sun-baked building brick in the Egypt of the pharaohs; to this day straw is used for thatching of roofs and/or walls in housing of indigenous, traditional design in parts of The Soviet Union, Africa, Japan, Iraq, Great Britain, and elsewhere.
The use of straw as an ingredient in modern, engineered construction of buildings probably dates from about 1930, with the invention of paper-faced construction panels of compressed, heat-treated strawboard by Niels Ryberg, in Sweden.
One company, now headquartered in Great Britain, but having many subsidiaries, affiliates or licensees in various countries, Stramit International, Ltd., of Creeting Rd., Stowmarket, Suffolk, England (also Stramit Corporation Limited, of Edmonton, Alberta, Canada), must be acknowledged to have been one of the earliest producers of such strawboard, under the trademark Stramit, and the literature which they have produced continues to be a valuable fund of background information about the physical characteristics of such strawboard and techniques and accessories useful for incorporating such strawboard in buildings. Another provider of such strawboard for use in construction of buildings was Tetratech Building Systems International, Ltd., which called its product Tetraboard.
A way for making strawboard that is useful in the building architecture of the present invention, including detailed descriptions of physical characteristics of a preferred, suitable strawboard, is disclosed in the Dvorak, U.S. Pat. No. 4,451,322, issued May 29, 1984, the entire disclosure of which is incorporated by reference herein.
To the knowledge of the present inventors in prior art uses of strawboard in the construction of buildings, the individual panels of strawboard have been used, whether singly, or with jointed construction, to fill the facial area between adjoining posts, beams, rafters, joists and similar elements of a separate (and heretofore believed necessary) structural frame or skeleton of a building. For instance, Stramit product literature describes use of its strawboard as panels for roof decking, roof insulation, interior wall lining, ceilings, and partitions, both fixed and movable. Tetratech product literature describes use of its strawboard as panels for these same uses, and as exterior sheeting, in fill panels, sub-flooring, sound attenuating panels, acoustical baffles and in the fabrication of doors. Probably because the aforementioned patent of Dvorak is concerned with apparatus for making strawboard, it does not contain an extensive description of uses for the strawboard other than mentioning that it is an architectural structural material that is versatile, durable, relatively inexpensive, and of considerable utility in the construction of dwellings and other buildings, in which it may serve as a ceiling or wall board, as a thermally insulative layer or as a material useful in acoustic absorption or isolation.
Some older Stramit product literature discloses the use of a strawboard in the construction of the walls and roof of a temporary bunkhouse. Recent Stramit product literature discloses use of strawboard in the construction of modular housing. In all of these instances either the strawboard is disclosed to be used for plating a balloon frame made of wood and/or fabricated sheet metal framing elements and/or to make use of metal structural framing elements built-into the strawboard panels, e.g. as U-shaped sheet metal channels clamped around the edge margins of the individual strawboard panels. In erecting a structure using such panels, mechanical connections are made between the metal channels of adjoining panels, thus connecting the panels together while simultaneously erecting a supporting framework.
The present inventors believe they have devised an invention which radically differs from the prior art described above, in that it calls for uniting panels of strawboard into a structural membrane, providing a building with substantially less use of any framing, so that all loading is primarily born and distributed by the relatively homogeneous strawboard, much as if it were an igloo made of strawboard and adhesive, rather than of snow and ice.
For those who are not familiar with the characteristics of strawboard such as that which may be used in practicing the present invention, a brief description will be provided here, although for more extensive information, the interested reader will certainly wish to consult the available literature.
In manufacturing strawboard, a suitable straw, of the same sort which is traditionally used as roughage and bedding for cattle, horses, sheep and the like, e.g. including any proportions of dry (typically less than 15 percent moist, by weight) stalks of the cereals (such as rice, wheat, rye, oats and barley), grasses, sugar cane bagasse is cleaned of foreign matter such as stones and clods of soil, as well as of fine particles and dust, and is fed at a uniform rate and well-distributed manner into the ram of an extruder, where it is shaped, compressed and baked, at a temperature of about 350.degree.-400.degree. F., continuously emerging as a billet of indeterminate length and a uniform thickness and width. Two, three and four inches are desirable thicknesses, and four feet is a standard width. The emerging board is typically golden in color. No adhesive generally is needed for sufficiently unifying the bulk of the board, since, during the extrusion process, natural constituents of the straw, such as lignins which typically make-up from about 10 to about 30 percent of its weight become activated and naturally adhere the constituents of the board together. Additional glue could be added as the straw is being fed to the extruder, as is done in the manufacture of particle board, but presently such is not thought to be necessary, and is not preferred. The same holds true for additions of anti-fungal agents, anti-bacterial agents, mold-inhibiters, rodenticides and the like, either as ingredients or as coatings.
Due to the action of the ram of the extruder, the grain of the bulk of the board typically runs crosswise and thicknesswise, although there are fiber interconnections running in all directions.
The emerging board preferably is wrapped first on one face and both edges, then on the other face and overlapping both edges, with paper, which may be any of the same types of paper as are commonly used for wrapping the cores of gypsum or foamed plastic wallboard. However, in the instance of strawboard manufacture, no attempt is made to particularly or significantly prestress the skin of the product e.g. by maintaining the paper under strong tension as it is adhered in place. Gray liner paper or brown Kraft paper, pre-sized as for painting and typically up to 0.06 inch thick is used as the covering of the core of the board, this covering being adhered in place using a suitable adhesive, e.g. urea-formaldehyde thermosetting resin adhesive. The resulting board generally is of a simple homogeneous material. That is, the only material besides straw making up the board is paper covering, which covering simply encloses the homogeneously distributed straw and does not add any meaningful rigidity of similar structural quality to the board.
After the continuous board is so covered, typically it is cut crosswise into sections of desired length, e.g. into panels each eight feet in length. Cut ends are covered by similar paper strips, similarly adhered in place.
The resulting panels have a density of about 16 to about 23 pounds per cubic foot and a modulus of elasticity of about 17500-21500 p.s.i., e.g. for a 3 inch thick panel. Such a panel typically has a longitudinal crushing failure (on a uniformly-loaded cross-section that is 47.25 inches wide and three inches thick, and a board density of 16.0 pounds/cubic foot at the beginning of the test), of approximately 6000 pounds, column failure of an eight foot tall panel of the same size and constituency typically being approximately half that figure.
Although unconsolidated natural straw is notoriously combustible and a fire hazard, strawboard of the type described herein chars when subjected to torching, but does not support combustion and generally self-extinguishes upon withdrawal of the torch. However, the paper covering can be combustible and a means for spreading flame, so, for meeting noncombustible construction requirements, it may be necessary to use covering paper which has been treated with a suitable flame retardant or the like.
Typically, the strawboard gains only one-thousandth in linear dimension upon being raised in ambient humidity from 40 to 90 percent. However, such strawboard is not itself waterproof and must be suitably protected if it is to endure a moist environment.